Organic stir-in pigments are known and are described for example in U.S. Pat. No. 5,554,217. These known stir-in pigments and stir-in pigment compositions show excellent stir-in properties when applied in paint and ink systems, particularly solvent-based systems. They have the great advantage that they can be incorporated into an ink or paint system by simply stirring the pigment powder into the paint system without the time and energy-consuming dispersion step in a bead mill.
It is also known to use certain pigment preparations, also called master batches, having a pigment concentration of 30 to 60 weight percent based on the preparation, as stir-in pigments. Such stir-in preparations show excellent properties in certain applications. However, they do have the disadvantage of employing an expensive preparation process, and occasionally they show incompatibility in some application media due to the high amount of carrier additives.
Many patents describe processes for pigment surface modifications to enhance certain pigment properties like color strength or rheological properties, for example by the addition of pigment derivatives such as pigment sulfonic acids, sulfonamides or other pigment derivatives. However, such treated pigments need to be dispersed for many hours in a bead mill in order to incorporate the pigment into a paint vehicle.
U.S. Pat. No. 3,806,464 discloses the encapsulation of pigments with an acrylic interpolymer, and Japanese published patent application SHO 57-49664A describes a process for the preparation of pigment compositions containing thermoplastic resins which can be dissolved in water with a basic substance. Although these pigments contain polar polymers, they are different from the present inventive pigment compositions because they contain different kinds of additives. Furthermore, they show incompatibility in certain application media. Therefore, their use as pigment compositions has not been wide-spread.
Other patents, such as U.S. Pat. Nos. 5,274,010 and 5,401,780, describe a process for warp-free pigmenting of polyolefins by coating the pigment surface with a polyvinylalcohol or other polar polymers such as acrylic polymers, cellulose derivatives, maleic anhydride-styrene copolymers and polyvinylpyrrolidone, alone or in combination with silane and zirconium acetylacetonate films. The coating of the pigment is achieved by (A) absorption of a preformed, soluble polymer onto the pigment or (B) by polymerization of a corresponding monomer in the presence of the pigment. In either case the resulting coated pigment is recovered by filtration from an aqueous suspension. Such suspensions are difficult to filter and can show strong bleed-through of very finely divided pigment particles. Therefore filter aids are often added to improve the filtration rates. However, the presence of these filter aids in the pigment composition can lead to incompatibility of the resulting coated pigments when applied in substrates other than polyolefins. Additionally, such pigment suspensions contain a large quantity of unabsorbed polymer which is dissolved in the aqueous medium and thus remains in the filtrate after filtration. This highly colored filtrate then requires special treatment in order to recover or degrade the soluble polymer and to decrease the filtrate's color to environmentally acceptable levels.
U.S. Pat. No. 5,145,524 discloses surface modification of pigments by treatment with polyvinylalkyl ethers to obtain pigments with enhanced properties for automotive finish systems. This surface treatment also improves the flow properties of the pigment in aqueous systems, leading to aqueous pigment dispersions with a higher solids content for direct application in waterborne paint systems. Such pigment dispersions can have the drawback of foaming and settling. Additionally, they require the addition of antifouling agents. Moreover, in most cases they still need to be dispersed in a bead mill for optimum dispersion.
U.S. Pat. No. 3,904,562 describes a process for encapsulating finely divided organic pigment particles with a polymeric outer layer by a precipitation process. This process is costly and environmentally unfriendly due to the use of a large quantity of an inorganic salt to induce precipitation of the polymer from the aqueous medium. Additionally, the particles encapsulated according to the teachings of this patent require a higher molecular weight polyvinylpyrrolidone and a larger amount of polymer based on the pigment to ensure complete encapsulation, which can adversely affect the compatibility of the pigment when it is applied in various different substrates.
Japanese laid open patent application SHO 58-208351 describes pigment compositions comprising a pigment and a water soluble copolymer of an ethylenically unsaturated sulfonic acid compound and a N-vinylpyrrolidone compound. Said pigment compositions can be incorporated into aqueous vehicles by simple mechanical stirring without a dispersion step. However, due to the presence of the polymeric sulfonic acids, these pigment compositions may not be suitable for many applications, for example, application in automotive paint systems, since it is well known that the strongly polar polysulfonic acids have a detrimental effect on the curing of the coating system. Additionally, the presence of the polysulfonic acids can enhance water absorption, which can further damage the weatherability of the pigmented coating system.
Due to environmental concerns and new environmental regulations the paint industry is moving extensively from solvent borne to aqueous borne coatings systems. Newly developed improved aqueous polymeric binder systems are helping to speed up this transformation, but they also force pigment manufacturers to continually modify their pigments to adapt the pigment properties to the latest customer needs.
The present invention relates to new stir-in pigment compositions in the form of microgranules comprising 85 to 99.5 parts by weight of a pigment and 0.5 to 15 parts by weight of an additive which comprises a vinylpyrrolidone polymer or copolymer. These new microgranule stir-in pigment compositions are obtainable by spray drying a high solids content aqueous pigment dispersion containing the pigment and the additive. They manifest enhanced pigment properties when applied in aqueous ink and paint systems, particularly those used in industrial coatings, trade sales and the automotive industry.
Unexpectedly, it was discovered that such stir-in pigment compositions can effortlessly be incorporated into aqueous ink and paint systems as a stir-in pigment without carrying out a dispersion step in a bead mill. Compared to conventionally dried and micropulverized pigments, the new pigment compositions are generally less dusty and can be more easily handled. They are also easily wetted and disperse rapidly to form homogeneous aqueous pigment dispersions which are practically free of larger pigment aggregates. Many small particle size pigments are known to aggregate during the drying process. However, even with pigments exhibiting this aggregation tendency, surprisingly, the inventive microgranular stir-in pigment compositions are easily dispersible in aqueous paint systems. Moreover such stir-in pigment compositions have the great advantage that they are compatible with a variety of aqueous paint and ink systems, including the most demanding automotive paint systems. Additionally, the new stir-in pigment compositions are obtainable by a very simple, economical and environmentally friendly method in conventional equipment.